(1) Field of the Invention
This invention relates to a vacuum-type circuit breaker, and more particularly to the composition of a material for making contacts used with the circuit breaker of the type described.
(2) Description of the Prior Art
In vacuum-type circuit breakers, to increase a dielectric strength raises an important problem which will call for special efforts to solve for many years to come. If the dielectric strength of a vacuum-type circuit breaker can be increased, the voltage that can be impressed on the circuit can accordingly be increased.
The problem of increasing the dielectric strength of vacuum-type circuit breaker has hitherto had two approaches. One is through increasing the gap between the two contacts and the other relies on selecting the most suitable components and their proportions for a material for producing the contacts. Heretofore, the latter approach to the problem has generally been adopted. This is because the former approach not only makes it impossible to obtain an overall compact size and a light weight in a vacuum-type circuit breaker but also difficulty is experienced in increasing the speed at which contacts are brought into or out of engagement with each other. The latter approach is free from these disadvantages.
Generally, copper which has high conductivity is used as the main component of a material for producing contacts because it is necessary to increase the ability of the contacts to interrupt a current or to increase the value of a current that can be interrupted. Thus, in increasing the dielectric strength of contacts by selecting the most suitable components and their proportions for the material for producing the contacts, a metal or metals which are themselves effective in increasing the dielectric strength are added to copper. There has been no established theory for increasing the dielectric strength, but it is believed that, qualitatively speaking, metals of a high melting point, of low vapor pressure and of high mechanical strength have the effect of increasing the dielectric strength when added to copper. Based on this idea, a lot of alloys have been produced and tested for dielectric strength. It must be admitted, however, that only a few of the alloys tested have been successful in accomplishing the object. In this connection, it must be pointed out that impurities or gas contained in the alloys and the microstructure of the alloys exert influences on dielectric strength which are not negligible. The alloys that have been most successful in the past are copper alloys containing either iron or cobalt. These alloys have a microstructure in which the phase of iron or cobalt having a melting point and mechanical strength higher than those of copper is dispersed in the matrix of copper. By using these alloys for producing contacts, vacuum-type circuit breakers having a rated voltage of 30 KV have been manufactured. However, there is a strong demand for contacts which have a higher dielectric strength.